The present invention relates to an optical lens to be provided on an information recording and reproducing device capable of conducting at least one of recording of information on an optical recording medium and reproducing of information recorded on an optical recording medium, and to an information recording and reproducing device equipped with the optical lens.
There has been available an optical pickup device (an information recording and reproducing device) as a device to conduct recording of information on an optical recording medium and reproducing of information recorded on an optical recording medium. The optical pickup device is one that converges a ray of light emitted from a semiconductor laser light source on an information recording surface of an optical recording medium with an objective lens (optical lens), and thereby conducts recording and reproducing of information.
On this objective lens, there is provided an antireflection coating, for improving utilization efficiency of light. The antireflection coating is generally formed in a way that a thickness of the coating at a certain position becomes thinner as that position moves from the central portion of the objective lens to the outer peripheral portion thereof, and the layer thickness is established so that the reflectance of the objective lens for light entering vertically the objective lens at its central portion may show the minimum value for the wavelength of the laser beam of the optical pickup device. Namely, the layer thickness of the antireflection coating is set so that an amount of light passing through the central portion may become the greatest.
Incidentally, in the antireflection coating, it is known that the wavelength-dependency of the reflectance for light is shifted more toward the short wave side as an angle of incidence of light grows greater. In the objective lens, an angle of incidence of light grows greater as a position of the incidence moves from the central portion to the outer peripheral portion. In the conventional objective lens, therefore, the wavelength-dependency of the reflectance is more shifted to the short wavelength side than the wavelength-dependency of the reflectance at the central portion, resulting in that the wavelength which makes the reflectance of light entering the outer peripheral portion to be minimum was shorter than the wavelength that makes the reflectance of light entering the central portion to be minimum. Therefore, in the objective lens on which the conventional antireflection coating is provided, the reflectance for a laser beam at the outer peripheral portion is higher though the reflectance for a laser beam at the central portion is low, thus, an amount of light transmitted through the outer peripheral portion becomes smaller relatively than an amount of light transmitted through the central portion, resulting in problems of deterioration of spectral intensity of transmitted light of the total lens, an increase in a beam spot diameter caused by a fall of light-converging functions and of a decline of an amount of beams.
In recent years, there have been made attempts to diminish a diameter of a light beam spot, namely, to diminish a beam spot sufficiently with an objective lens, so that recording and reproducing of information by light may be conducted under the condition of high density recording, for realizing a large capacity of an optical recording medium. Since the beam spot diameter is in inverse proportion to a numerical aperture (NA: Numerical Aperture) of an objective lens, a technology for high NA of an objective lens has been advanced. Recently, a lens wherein an angle made between a normal line on a surface of an effective diameter of the lens and the optical axis is 45° or more, further, 55° or more has come to be used.
However, in the objective lens having high NA, an angle of incidence of light on an outer peripheral portion of the lens is extremely large, because a curvature of the lens surface is great. Therefore, an amount of light transmitted through the outer peripheral portion has been lowered sharply, resulting in problems that an increase in a spot diameter cannot be controlled, and realizing of a large capacity of an optical recording medium is disturbed.
As a technology to solve the problems stated above, there are disclosed technologies to increase an amount of light transmitted through the total lens by increasing an amount of light transmitted through the outer peripheral portions (for example, see Patent Documents 1-3).
(Patent Document 1)
TOKKAIHEI No. 10-160906
(Patent Document 2)
TOKKAIHEI No. 11-222446
(Patent Document 3)
TOKKAI No. 2001-6204
However, when increasing simply an amount of light transmitted through the outer peripheral portion by the technologies disclosed in the Patent Document 1-3, problems of deformation of a beam shape, deterioration of jitter characteristics and of an increase of cross talk are caused, and there has been a possibility of a decline of recording and reproducing functions. As stated above, it has been difficult to optimize a balance between light-converging functions and an amount of light.
In the case of an objective lens of an optical pickup device employing laser beams with two or more types of wavelengths, in particular, it has been extremely difficult to optimize a beam shape while increasing an amount of transmitted light for the rays of light covering all working wavelengths. Incidentally, as the optical pickup device of this kind, there are given an optical pickup device for DVD/CD employing light with wavelength 660 nm and light with wavelength 785 nm, for example, and an optical pickup device for high-density-optical-disc/DVD/CD employing light with wavelength 405 nm, light with wavelength 660 nm and light with wavelength 785 nm. The high density optical disc in this case means those, for example, a blue ray disc and AOD (Advanced Optical Disk).